1. Field of the Invention
The present invention relates to tailoring neighbouring base station information transmitted in a first cellular telephone network on the basis of the relationship between a subscriber and a second network.
2. Related Art
The present invention will be described with reference to the GSM (Global System for Mobile communication) standard because the skilled person is familiar with this standard and will, therefore, more easily be able to understand how to implement a system according the present invention. Furthermore, details of conventional GSM are readily available from publications such as Mouly, M. et al, xe2x80x9cThe GSM System for Mobile Communicationsxe2x80x9d, ISBN 2-9507190-0-7. However, the present invention is not merely a variant of GSM and may be applied to other cellular mobile telephone systems. Accordingly, terms having particular meanings within GSM are not to be construed as limiting the scope of protection sought to GSM systems.
GSM provides for a plurality of service providers to make available respective public land mobile networks within an area covered by the same regulatory authority. Typically the regulatory authority is a national PTT (Post, Telephone and Telegraph) authority.
Roaming between public land mobile networks in the same area may be provided by service providers. Assuming that a subscriber is able to roam between two networks, a choice needs to be made between the available networks. The subscriber""s mobile station could scan all frequencies and register with the network having the strongest signal. This unfortunately is time consuming and very demanding on power. Consequently, in order to maximise battery life, GSM900, phase 1 adopts the practice of the serving base station transmitting a list of beacon identities of neighbouring base stations, and the mobile station only scans these identities. This approach is not optimum from the point of view of service providers because once a subscriber has roamed to another network, for instance as a result of a discontinuity in the coverage provided by the subscriber""s home network, the subscriber will not return to his home network until the subscriber manually instructs his mobile station to register with his home network, he enters a discontinuity in the coverage of the current network or switches his mobile station off and then on again.
This is particularly undesirable when the home network is a private network, on which additional services or reduced call changes are available to its users. The private network may comprise a single base station, as is disclosed in International Patent Application WO98/03002. In the system described therein, a dual-mode cordless/cellular telephone (for example operational on the DECT and GSM standards) can have incoming calls, initially directed to the fixed telephone line associated with the cordless function, diverted to the handset""s cellular identity when the cordless base station cannot detect the presence of the handset. However, if the handset is carried into or out of range of the cordless base station while a call is in progress, the call will continue on the system in which it was initiated, or fall if radio contact is lost on that system.
International Patent Specification WO94/06220 discloses a system in which there is a supplementary network whose base stations have a dedicated range of frequencies separate from that of the main network. Mobile stations having access to the supplementary network are arranged to scan these frequencies and use such base stations when available. However, as already discussed, this is very demanding on power and is time consuming. Moreover, it requires the supplementary network and main network to operate on distinct frequency ranges. This requires specialised mobile units capable of operation in both ranges, and precludes efficient frequency re-use. For example, International Patent Specification WO97/14261 discloses a system in which separate handover arrangements are provided for single-band and multi-band mobile units, in areas where some bands are not universally available.
According to a first aspect of the present invention, there is provided a method of operating a cellular telephone system having a subset of base stations only available to some subscribers to the system, the method comprising:
transmitting, to each mobile station, neighbouring base station data for constructing a list of beacon identities for base stations neighbouring the mobile station""s serving base station, the transmission being made in a control channel, dedicated to the particular mobile station, wherein the list of beacon identities includes members of the said subset only if the subset is available to the mobile station""s subscriber,
receiving signal reports from the mobile station for the plurality of bass stations, and selecting a base station for use by the mobile station based on said reports and on whether the base station is a member of the subset,
the handover being controlled such that mobile stations for which access to the subset of base stations is permitted are not handed over to base stations not forming part of the subset unless the signal reports for the base stations indicate that reliable communication cannot be effected between the mobile station and any member of the subset.
By selecting the neighbour list according to the user identity, private cellular networks may be defined using the same frequency bands as the main network, with standard mobile units having permission to use the private network able to switch between systems automatically, but preferentially using the private network.
xe2x80x9cBeacon identityxe2x80x9d means the identity of a beacon on which a mobile station can meaningfully make signal strength/quality measurements. The neighbouring base station data may itself include beacon identities in the form of their actual frequency in Hz, kHz, MHz, rad/s etc. or a channel number. Alternatively, the neighbouring base station data may comprise data from which the mobile station can derive the beacon frequency, e.g. by an algorithm relating a base station identity code to beacon frequency. Similarly, the beacon frequency list need not literally comprise a list of frequencies. Rather the list must merely contain data sufficient for the mobile station to tune to the beacon, for example the scrambling code of a CDMA (code division multiple access) system.
In GSM, the use of SIM (subscriber identity module) cards means that the system features available to a subscriber are largely independent of the particular mobile station because they are governed by the SIM, which may be in many different mobile stations. However, the present invention is also applicable to systems in which a subscriber""s identity corresponds to a mobile station""s identity.
The base stations of the subset may be those of one network whilst those not in the subset may belong to one or more other networks. These networks may be physically distinct or logically distinct. Logically distinct networks may use the same hardware, for instance in the case of a virtual private network provided by a public land mobile network operator.
The present invention may be used to assist in the integration of a private cellular mobile telephone network and a public cellular mobile telephone network. However, it can be applied to the solution of many problems, including, but not exclusively so, handover between public networks at national borders for subscribers with roaming privileges. In this case, the mobile station of a subscriber with roaming privileges would be informed of beacon identities of the base stations of the network privileges would be informed of beacon identities of the base stations of the network in a neighbouring country when approaching the border in dedicated mode. Handover to a foreign network would only take place if signal quality on the home network (measured either as an absolute value or relative to the foreign network) fell below a specified threshold. Handover back to the home network would take place if signal quality on that network returned above that threshold.
Preferably, the transmitted data is configured for the generation of a beacon identity list including a beacon identity for a base station in said subset. This will occur when the subscriber has access privileges to base stations in the subset. The access privileges may be determined ad hoc. For instance traffic levels in the subset base stations may be taken into account with the result of the beacon identity of the base station in the subset not being transmitted, if the traffic capacity of that base station has been reached.
The cells, or coverage areas, served by the base stations in the subset, which may be a private network, may be much smaller than those not in the subset, which may constitute a public network. The small cells may, for example, cover a large industrial site wholly within a cell of a public network. Consequently, a xe2x80x9cneighbouringxe2x80x9d cell, belonging to the subset, may be located wholly within a cell not forming part of the subset. The areas of coverage of neighbouring base stations may be adjacent or overlap, or one may be included entirely within another.
The invention may also be used to allow handover of a suitably equipped mobile telephone, with suitable access rights, between different radio technologies, for example the mobile telephony standards known by the acronyms GSM, PCN (Personal Cellular Network), PDC (Personal Digital Cellular), UMTS (Universal Mobile Telecommunications System), and DECT (Digital European Cordless Telephony). Note that the technologies need not all be cellular (although at least one of them must be cellular): it is only necessary that a cellular network can have beacon identities of another system added to its neighbour list. The invention may, for example, be applied to a system such as that disclosed in the aforementioned International Patent Application WO98/03002, by adding the cordless base station to the cellular network""s neighbour list for the dual purpose handset associated with that base station.
A method according to the present invention preferably comprises:
maintaining a record of the neighbour relationship between a first base station in the subset of base stations and a second base station not in the subset,
determining whether the mobile station""s subscriber, when the mobile station is located in said first base station of the first network, has an access-granting relationship with the base stations of the subset, and
transmitting the data identifying the beacon identity of the second bass station in said dedicated control channel if the mobile station""s subscribe has said access-granting relationship with the base stations of the subset.
However, other factors may be taken into account. For instance traffic levels in the second network may be taken into account with the result that the beacon identity of the base station of the second network is not transmitted if the traffic capacity of that base station has been reached.
The neighbouring base station data may be arranged to allow generation of a standard beacon identity list which is extended for the appropriate mobile stations to include a beacon identity of a base station in the subset. Alternatively, the subset base station data may replace non-subset base station data transmitted to other mobile stations.
A mobile station receiving a list transmitted in accordance with the present invention, will typically report signal measurements for the transmitted beacon identities. These reports can then be used for controlling handovers.
A handover of a mobile station currently served by a base station not of the subset to a base station of the subset may be initiated if the signal report for that base station indicates that reliable communication can be effected with the mobile station using that base station. Going the other way, preferably, handover of a mobile station currently served by a base station of the subset to a base station not of the subset, even if its signal reports are superior to those of the subset base stations, is only initiated if the signal reports indicate that reliable communication can not be continued using any base station in the subset.
According to a second aspect of the present invention, there is further provided a fixed cellular mobile telephony apparatus configured to operate according to the method according to the first aspect of the invention.